Fix jira labels importer response handling

What does this MR do?

We introduced importing labels from Jira. First, we import all labels and then during the issues import we either use imported labels or create them on go in case the first step failed.

Unfortunately, right now the first step will always fail as we don't handle the response well. I am not sure how this started as I am pretty sure I tested importing the labels using the service but it is possible I changed the working solution when I was writing the specs.

The problem is that jira_client returns already a json response. We, however, were using Webmock in specs where json string needs to be returned. I am not sure what exactly is happening inside the Jira client and how it communicates with Jira as I think it should handle webmock response 🤔 anyway, now the mock is correct and the Importer is fixed.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.